Answer
$ 1.0 \times 10^1 \space moles \space H_2$ or $ 10. \space moles \space H_2$ are required to react with 5.0 moles of $O_2$.
Work Step by Step
1. Determine the conversion factor between $O_2$ and $H_2$ moles:
According to the coefficients: 2 moles $H_2$ = 1 mole $O_2$ .
$\frac{ 1 \space mole \space O_2}{ 2 \space moles \space H_2}$ and $\frac{ 2 \space moles \space H_2}{ 1 \space mole \space O_2}$
2. Use this conversion factor to calculate the amount of $H_2$ moles:
$5.0 \space moles \space O_2$ $\times$ $\frac{ 2 \space moles \space H_2}{ 1 \space mole \space O_2} = 1.0 \times 10^1 \space moles \space H_2$
